Some pieces of electronic test equipment have an aligned array of connectors on a front (or perhaps rear) panel. For example, an oscilloscope may have four vertical input channels and one auxiliary or trigger input whose BNC connectors are on the front panel and uniformly spaced along a line. A further characteristic is often that the bayonet pins are required to have a particular orientation with respect to the rest of the equipment. For example, the bayonet pins might need to be aligned with the vertical (or perhaps horizontal) axis of the cabinet. There are various reasons that may cause this, ranging from simple aesthetics to electromechanical cooperation with accessories. A high bandwidth ""scope may accommodate an active probe that has a housing containing not only the vertical signal input connector, but ancillary electronics and a fair number of other electrical connections made to the ""scope by spring loaded pins. The orientation of the housing after being attached to the front panel is important, then, to ensure proper connections for those pins. That orientation may well be affected by the direction of the bayonet pins for the connectors on the front panel. As another example, some pieces of test equipment have multiple (say, female BNC) inputs that receive an accessory or test fixture that attaches to all those inputs at once. Rather than use a collection of short cables with male BNC connectors on their ends, it might instead be preferable to locate the male connectors on some surface of the accessory or fixture, and do so in a rigid pattern corresponding to that of the female connectors on the test equipment front panel to which they are to be attached. This allows the attaching of the accessory or fixture to be a simple unit operation.
This is all well and good, and seems harmless enough, but it turns out that it can cause a definite degree of aggravation during manufacture. Consider a high bandwidth oscilloscope. For performance reasons, it is common that the connectors be cross series adapters, say, BNC on the external side and perhaps SMA on the internal side. A short length of hard or semi-rigid coax with a loop or xe2x80x9cSxe2x80x9d bend therein (or perhaps flexible coaxial cable) connects the SMA connector to a motherboard that is in turn carried by a chassis. The BNC ends of the connectors protrude through holes in a casting (or other substantial portion of a machined panel) that is also mounted to the chassis. Besides offering mechanical strength and intended spacing, the casting or panel often is used, perhaps in conjunction with an additional RF gasket, to provide a good RF ground for shielding or suppression of Electro-Magnetic Interference. (In the event that the connector is part of a true transmission line, the casting or panel is not part of it; it is just well connected to the outer shield of that transmission line.) During manufacture of the motherboard it is convenient if the attachment for the short lengths of coax are soldered to the board at the same time that all the other components are soldered on. We should also like the front panel connectors to already be on those lengths of coax. It is almost a certainty that it is not practical to have the front panel itself present during that soldering operation (it would be there with the intent that it would hold the coax and connectors in their proper positions), and something needs to accomplish that function if we are determined to proceed with the coax and connectors being present during soldering.
One aggravation that arises during such construction is the need to resist the tendency of the connector body to rotate during the tightening of the nut that holds it against the front panel. This is necessary if the bayonet pins are to remain in an intended orientation, since reaction forces tend to rotate the connector during the tightening process. This is a definite problem, even if there is no preferred rotational orientation, since the motherboard ends of those connector/hard coax combinations are often soldered to the motherboard prior to their being affixed to the casting. If the induced rotational force is not resisted it stresses the solder joint, and also lets the bayonet pins move from their intended orientation. This has led to the use of an external assembly fixture that temporarily holds the connectors aligned while the solder is applied, and another that keeps the pins oriented while the nuts are being tightened. If this were not done the solder joint to the motherboard would act as a wrench keeping the connector in place while the mounting nut was tightened, which would be most undesirable.
Another aggravation arises because the spacing between the connector/coax pads on the motherboard and the spacing between the holes in the casting or machined panel are not always identical. This places an extra demand on the fixturing, since it must accommodate that, as well as prevent rotation. Nor is it only variations in connector-to-connector spacing that can produce mis-alignment. Variations in sheet metal parts and in the way the motherboard is laid out or, especially, is trimmed, can produce a translation between the connector spacing on the board and the spacing for the holes in the front panel, which results in a mis-alignment, even when both sets of spacing are correct.
Fixtures that mitigate all these aggravations are not always inexpensive to develop, manufacture or maintain. And there is the subsequent issue of field repair of the test equipment. The technicians in the field will almost certainly not have the fixtures, or may not fully appreciate all the ways that things can go wrong, with the result that their repairs will put expensive motherboards at risk. Sometimes those boards are worth thousands of dollars.
It would therefore be desirable if there were an inexpensive and effective way of aligning the connectors and their intervening rigid coax on the motherboard during soldering, and that would also later serve as a strain relief to prevent connector rotation during the tightening of the mounting nut against the casting or panel, even though the connector may need to shift slightly in one direction or another to account for differences in spacing. The technique should also cooperate well with the need to provide a good RF ground. And, it should be inexpensive and persist for later use by field service personnel. That is quite a wish list. What to do?
A solution to the problems of obtaining connector alignment between a motherboard mounted horizontally in a chassis and a front panel, of establishing an initial pin orientation (or angular position of another connector feature), of maintaining that orientation by preventing connector rotation during tightening, and of having these properties persist after initial manufacture and be available for subsequent field maintenance, is to attach a suitable sub-chassis bracket to the motherboard. The sub-chassis has holes: (a) through which the connectors pass and that space the connectors apart by an intended nominal amount; (b) that are shaped to allow the connectors to shift along an axis parallel to the planes of the motherboard and of the panel (i.e., horizontally) as needed to form the mechanical path from a particular spacing on the motherboard to a panel, with its own actual particular spacing, and to which panel the connectors are each fastened with a nut; and (c) that are shaped to prevent all but a small amount of connector rotation during tightening, and that pre-position the connectors rotationally so that after tightening they are indeed correct. The sub-chassis bracket holds the connectors, and the coax connecting them to the motherboard, in place while the motherboard has its components soldered thereto. The nuts attaching the connectors to the panel have a symmetrically tapered or curved surface on the side that contacts the outside of the panel, whose holes therethrough are somewhat oversize. The non-flat surfaces of the nuts individually center the connectors in their respective panel holes as they are tightened and become perpendicular to the panel. Horizontal shifting of the connectors in the sub-chassis bracket during tightening is accommodated in that the other end of each connector has already been connected to the motherboard by an intervening conductor that has a strain relieving loop or bend. These conductors, which may be lengths of either hard or semi-rigid coax, coaxial cable or even a wire, bend slightly to accommodate any horizontal shifting of the connectors. Vertical shifting is accommodated by flexure of the motherboard, which is broadly imparted and distributed over a large area by the size and stiffness of the sub-chassis bracket. To provide a particular bayonet pin orientation of the connectors, which may be of type BNC, and to prevent the connectors from rotating and disturbing that orientation during the tightening of the nuts, each connector is generally cylindrical in cross section and has a central region of increased diameter that forms a shoulder. There are two horizontal flats on this region of increased diameter. The shoulder bottoms out in a stepped hole in the sub-chassis, the larger diameter of which accepts the shoulder and is of a single or double xe2x80x9cDxe2x80x9d shape that is the complement of the increased diameter with the flats. This prevents all but a very small amount of connector rotation. The flats are essentially parallel with the horizontal direction, and thus cooperate with any needed horizontal shifting of the connectors. The flats may be pre-tilted opposite the direction that the connectors attempt to rotate during tightening, and by the small amount that they do so rotate, so that the final result is exactly aligned connector pins. It is also waterproof.